A residual image phenomenon on a display device mainly refers to that: when the display device is switched to display another image after displaying a same image for a long time, the display device may impose a residual image of the previous image on the new image. As for a liquid crystal display (LCD), the main reasons that cause residual images are as follows. Generally, as the absolute values of positive and negative voltage values of a common voltage applied to common electrodes of the LCD during polarity inversion cannot be exactly the same, movable ionic impurities in liquid crystal molecules are migrated and gathered to one electrode under the action of asymmetrical voltages during polarity inversion. Then, the ionic impurities can still produce internal electric field to rotate the liquid crystal molecules even after the driving voltage disappears, and hence residual images can be formed. Currently, a conventional residual image level determining method is to determine the residual image level of a sample image by visual observation by human eyes. This residual image level determining method has high subjective casualness and low accuracy, does not have a uniform standard for residual image level determination, provides different residual image level determining results by different people, and therefore cannot obtain objective results and may obstruct improvement of subsequent processes.